1. Field of the Invention
The present invention generally relates to an electro-mechanical transducer for converting an electrical signal into a mechanical displacement such as a so-called bimorph, and is directed more particularly to an electro-mechanical transducer which is fixed at its one end and carries at its other end a magnetic head and in which a mechanical displacement is generated so as to displace the magnetic head in response to a voltage applied to the transducer.
2. Description of the Prior Art
Recently, in order to increase the record density in a magnetic recording and reproducing apparatus such as a VTR (video tape recorder), such a test to make the width of a record track as narrow as possible has been attempted. In the VTR or the like in which the track width is narrow, the position of a playback or reproducing magnetic head relative to the record track requires higher accuracy. It is rather difficult in technical point of view or requires high cost to present the above positional relation of high accuracy by merely mechanical accuracy. Therefore, in the art an electro-mechanical transducer element is employed to control the positional relation of the magnetic head to the record track to be always at a predetermined relation. That is, the electro-mechanical transducer element is fixed at its one end to a magnetic head drum and carries a magnetic head coupled to its other end, and an electrical signal, which responds to the variation of a reproduced signal caused by the variation in the positional relation between the magnetic head and record track, is applied to the electro-mechanical transducer element to displace the other end thereof and hence to set the magnetic head to be always at a correct position relative to the record track.
In general, an electro-mechanical transducer element used in the tracking servo of the VTR must be displaced greatly by a low voltage. Especially, in a VTR with a wide track width, the electro-mechanical transducer must be displaced greatly such as for about several hundreds to six hundred micro meters (.mu.m).
As shown in FIG. 1, a prior art electro-mechanical transducer element consists of two piezoelectric plates 2, each having electrodes 1 coated on its both major surfaces and a plate such as a so-called shim plate 4 located between the piezoelectric plates 2 to be bonded thereto through adhesive agent 3. The piezoelectric plates 2 are each made of piezoelectric material such as ceramic, polymer, complexion of ceramic and polymer or the like, the shim plate 4 is made of metal such as titanium, stainless steel, phosphor bronze or the like, and the adhesive agent 3 is made of a conductive adhesive, respectively.
The piezoelectric plates are polarized to align the polarization in the direction perpendicular to the major surfaces. However, the directions of polarization are opposite with respect to each other in the piezoelectric plates.
When a voltage is applied across the outer electrodes 1 of the piezoelectric plates 2 so as to give them electric fields perpendicular to the major surface, one piezoelectric plate 2 expands while the other piezoelectric plate 2 shrinks, so that the piezoelectric transducer element is displaced. That is, as shown in FIG. 1, when one end of the electromechanical transducer element consisting of the laminated piezoelectric plates 2 and shim plate 4 is mechanically fixed or clamped, the other end of the transducer element is displaced as shown by arrows in FIG. 1.
However, the above prior art electro-mechanical transducer element is not displaced as much.
When a given voltage is applied across the electrodes 1 coated on both the major surfaces of piezoelectric plate 2 shown in FIG. 2, the piezoelectric plate 2 is expanded or shrunk in accordance with the direction of the electric field applied thereto. In this case, the expansion and shrinkage are generated with respect to x- and y- directions which are perpendicular with each other. Therefore, when the shim plate 4 made of metal whose modulus of elasticity or Young's modulus is equal with respect to both x- and y- directions or which has isotropy in Young's modulus is bonded to one major surface of the piezoelectric plate 2 to mechanically fix or clamp the same with respect to both x- and y- directions, bendings are generated in the piezoelectric plate 2 in both x- and y- directions, as shown in FIG. 3. Accordingly, if the displacement by the bending is only one direction, for example, x-direction is necessary, the bending in the y-direction rather prevents the bending in the x-direction in view of construction.
Further, in the electro-mechanical transducer element shown in FIG. 1, generally, polymer adhesive agent is used as the adhesive agent 3. In fact, however, the softness of this adhesive agent operates to prevent suitable clamp of the piezoelectric plate 2 and accordingly the generation of bending is suppressed.
Thus, it will be apparent that the prior art electro-mechanical transducer element can not present sufficient displacement.